it's not necessary to park car on petrol over night,
1. petrol needs lesser amount of ignition spark than that of CNG, battery factor especially in winters
2. depends on fuel pump
i) mechanical kindda stuff: the more u pump the more fuel goes to carb, could be trouble some when u have got few no. of attempts early morning
ii) elctronic: s/w on and fuel goes to carb, much better than former one
3. depends on fuel manageemnt sys
i) carby: it have above stated problems, probably one has to use manual choke to increase air/fuel mix in carb
ii) EFI: sprays fuel and no issues of carb and clogegd capillaries, moreover, ECU sets everything, for instance there is no manual choke in them
So bottom line is
1. if one has mech point sys, make sure point is clean, for CDI system no such worries though one can change the distributor easily
2. low RPM settings are fine while doing CNG settings
One more thing, watere connections are usually not done with CNG kits, thus in winters it can cause problems, actually thermostat valve opens and make the flow of radiator water possible around engine, sometimes ppl remove that thing so engine does not get optimum temp quickly so engine vibrations should be expected in this case, more noticeable in winters on CNG.